1. Field of the Invention
This invention relates to a method for manufacturing a photographic paper support having a resin coating thereon. More particularly, it relates to a method of making a resin coated photographic paper having improved adhesion between the resin layer and the paper surface.
2. Description of Related Art
In order to keep the processing solution from penetrating photographic paper base during the development steps, synthetic resins of the polyolefin type such as polyethylene and polypropylene are coated on paper. The side to be coated with photographic emulsion has inorganic fillers such as titanium dioxide to provide white background. The opposite side has generally a blend of low density and high density polyethylene for curl control purpose. Since polyolefins are nonpolar by nature, extra steps are required to promote good bond between a polyolefin and paper surface. One method is to oxidize the molten polyolefin curtain prior to the coating. Polymer melt temperature is kept as high as 338.degree. C. (640.degree. F.) to promote oxidation. But the high melt temperature produces unwelcome results such as polymer degradation and crosslinked gel formation. The distance between the die lip and the lamination nip can be adjusted to provide longer oxidation time. But too great distance can hurt oxidation because it will lower the melt temperature. The coating line speed can be reduced to allow more time for oxidation. But this is not attractive from the cost point of view.
Another method is to precoat paper with adhesion promoting chemical primers. However, because the photographic paper is a rather porous substrate, the priming solution soaks through the paper rather than staying on the surface. Also photosensitivity of chemical primers is another concern.
There are other means of treating substrate surfaces. U.S. Pat. Nos. 5,147,678; 3,892,573; 4,135,932; 4,729,945; 4,186,018; and 4,128,426 describe treating polymeric surface using flame, corona, or ozone to improve on adhesion. U.S. Pat. No. 4,481,289 describes treating paper surface with corona discharge and oxidizing the polyolefin melt curtain using ozone and air mixture. It claimed that, by this method, improved adhesion was observed at 183 m/min (600 FPM) line speed. A line speed of 183 m/min is rather slow in today's environment. There is a great need to increase the line speed to 457 m/min (1,500 FPM) or beyond.